A robust and broadband waveguide assembly for a microwave receiver front end is disclosed that should not be damaged by exposure to intentional or unintentional high power microwave energy. This assembly uses an electro-optic modulator inside the assembly that senses the microwave or RF electric field at that electro-optic modulator and produces a modulated optical signal. The assembly maximizes the transduction of the RF signal to an RF modulated optical signal by concentrating the electromagnetic field using a tapered ridged waveguide transition, terminating the ridged waveguide abruptly at the end of the fixture in an ostensibly open circuit, and placing the modulator at an electric field antinode of the resulting standing wave in the assembly.
The prior art includes:
1. An article by R. C. J. Hsu, A. Ayazi, B. Houshmand, and B. Jalali, entitled “All-dielectric photonic-assisted radio front-end technology”, Nature Photonics, vol. 1, September 2007, pp. 535-538. The disclosed apparatus uses an electro-optic modulator to sense the level of the microwave signal coupled into a dielectric resonator antenna. The electro-optic modulator is part of an RF-photonic link that provides electrical isolation between the antenna and the rest of the microwave receiver front-end (which contains the sensitive radio components), thereby achieving tolerance to unwanted high power electromagnetic radiation. This prior approach is based on an electro-optic microdisk resonator (acting as the modulator) that is intimately in contact with the dielectric resonator antenna. Because of its resonant antenna, this prior art apparatus couples in substantial power from incident radiation at only those frequencies that match the resonant frequencies of the antenna. Nevertheless, despite its all dielectric construction, the power at those resonant frequencies is slowly absorbed by the dielectric antenna of this prior art apparatus over a characteristic time that is inversely proportional to the resonator linewidth. If that absorbed power is sufficiently high, it can damage or alter the antenna and the modulator.
2. An Article by A. Ayazi, R. C. J. Hsu, B. Houshmand, W. H. Steier, and B. Jalali, entitled “All-dielectric photonic-assisted wireless receiver,” Optics Express, vol. 16, No. 3, Feb. 4, 2008, pp. 1742-1747. This article reports a LiNbO3 electro-optic modulator, in the shape of a disk optical resonator, which is coupled to a dielectric resonator antenna. Input and output optical fibers are coupled to the resonator modulator through the use of an optical prism. The nature of the optical disk resonator and the dielectric resonator antenna apparently limits this approach to bandwidths less than 10%.
3. Asif A. Godil, “Partially Loaded Microwave Waveguide Resonant Standing Wave Electro-Optic Modulator, U.S. Pat. No. 5,414,552, issued May 9, 1995.
In U.S. patent application Ser. No. 12/176,071, filed on Jul. 18, 2008 and entitled “Microwave receiver front-end assembly and array”, an RF front-end assembly is described in which a TEM horn antenna is transitioned into a TEM waveguide where an electro-optic modulator or an array of electro-optic modulators is placed. The TEM waveguide preferably is filled with a dielectric material whose dielectric constant is the same as the dielectric constant of the electro-optic modulator material. In contrast, this disclosure provides an alternative approach to this prior approach in that the presently disclosed invention shares an objective of concentrating the electromagnetic field to increase the electro-optic conversion efficiency of the electro-optic modulator without forming printed circuit electrodes directly on the modulator substrate. Also, it shares the objective of producing an electric field of uniform amplitude across the modulator. One advantage of this present disclosure is that the input of the disclosed assembly is a standard rectangular waveguide, which facilitates its connection to standard waveguide circuits and antennas. In addition, because the disclosed assembly uses a tapered ridged waveguide to concentrate the field delivered to the electro-optic modulator, the structure is contained within a solid rectangular waveguide frame, thus making it robust and easier to handle to the approach disclosed in U.S. Ser. No. 12/176,071.